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@mrohr After you asked on gitter, I did some digging in the reason why the tests was failing in IE11, it looks like Object.prototype.toString.call(source) (https://github.com/mrohr/angular.js/blob/d5dbb5973b4588e75afee9d81a0f008f69dc033d/src/Angular.js#L997) evaluates to [object Object] for both Map and Set in IE11, resulting in the usage of Object.create(Object.getPrototypeOf(source) (https://github.com/mrohr/angular.js/blob/d5dbb5973b4588e75afee9d81a0f008f69dc033d/src/Angular.js#L984) to copy instead of the map/set code.
This throws the exception you're getting when accessing size on the destination object:

Map.prototype.size: 'this' is not a Map object.

I created a fiddle to demonstrate the issue outside angularjs: https://jsfiddle.net/0mstLj6q/7/

Running this plunkr in IE11 (Windows 10) shows:

Running this in Chrome shows:

@frederikprijck Thanks for your help, I'm guessing this feature is pretty much stuck, unless we set up the tests to not run in browsers that don't print map/set objects .toString() as '[object Map]'/'[object Set]'?

Or are there alternative approaches available to detect Map and Set in IE11 (or all browsers)?

I just did some debugging and this validates to true in Chrome and IE11:

source.constructor === Map;
Object.getPrototypeOf(source) === Map.prototype;

aswell as

source.constructor === Set;
Object.getPrototypeOf(source) === Set.prototype;

There are a couple of problems with this feature (which are not specific to the feature itself, rather general problems with similar features):

1. In order to be able to reliably use the feature throughout AngularJS, other internal helpers need to support it too (e.g. equals()). Supporting a feature in copy() only, is useful when using copy() as a general helper utility, but this is not its purpose and adds extra size and maintenance overhead.

2. It is really difficult to ensure that the feature works reliably and consistently across browsers and browser versions, because it relies on JS features that are not supported by all browsers, are fairly new additions (which means browser implementations are buggy and may change frequently) and are only partially supported by some browsers (which makes it even more difficult to rely upon or polyfill).

For these reasons, I am going to put it in the Ice Box for now (but I am open to being convinced otherwise - now or in the future 😃).
In any case, thx for working on this @mrohr 👍